Our objective is to test the common assumption that the muscles doing most of the work during terrestrial locomotion function isometrically (at constant length). We hypothesize that during locomotion, muscles function isometrically only at preferred speeds, and at other speeds they undergo larger amplitude changes in length. Additionally, we suggest that isometric activity at preferred speeds produces a minimum in energetic expenditure (measured as Cost of Transport - Cr), and, muscle length changes trigger gait transitions. "Preferred speed' refers to the behavioral tendency for animals to use only a limited set of speeds near the middle of the much broader range of speeds they are capable of using in a particular gait. Preferred speeds have been observed in a variety of different settings, but they are the least well understood of all locomotory behaviors. Our Specific Aims are: 1) Confirm that the trot- gallop transition sped and preferred trotting speed are lower when horses locomote on an incline or carry a load; 2) Compare muscle function, stride kinematics, and the cost of transport when trotting and galloping on the horizontal and on a 10% incline, and 3) Compare the same parameters when trotting and galloping on the horizontal and when carrying a load equaling 20% of body mass. Horses will be used because they are easily trained, readily exhibit preferred speeds, and their large size facilitates the measurement of muscle length. We will measure: changes in muscle fiber length as a function of speed (sonomicrometry), muscle activity (EMG), preferred speeds, gait transition speeds, and Ct (respirometry). This research will contribute to our understanding of the function of muscle during locomotion and the results should be generally true of all terrestrial locomotion, bipedal as well as quadrupedal. These experiments will be ideal for MBRS students because the animals are fascinating, the questions interesting and understandable, and most of the techniques and data analysis can be quickly learned. This research does not require an extensive course background, so students can become involved early in their academic careers when there is the best chance of interesting them in careers as biomedical researchers.